The present invention relates to a monitor unit for sensors, and particularly ultrasonic sensors for monitoring the backup of a motor vehicle. The sensor has an element for influencing the sensor in a predetermined manner for test purposes.
Such monitoring units are known, for example, from U.S. Pat. No. 4,561,064. This prior patent describes a range measuring device having a transmitter and receiver unit for a measuring signal reflected by the object being measured. A transit time determination detector analyzes the measured signal and generates, upon reaching at least on preset travel time, a predetermined acoustical and/or optical distance signal. The transmitter and receiver unit has a reflector assigned to it at a predetermined distance. The travel time signal received by this reflector drives an acoustical and/or optical function control unit. The range measuring device can operate using ultrasound or infrared light. Active function testing and function display takes place continuously during the operation of the range measuring device.
The foregoing operating mode has several disadvantages. The test object, i.e. the reflector, may hinder the sound transmission. Furthermore, the precise measurement of the range of the object is influenced by multiple reflections from the reflector.